feat: confirm before leaving a game, and fix untranslated strings

Exit confirmation
- A dialog in the app's own style now guards every in-game screen. The safe
  action is the prominent one, so a stray tap keeps you in the game.
- Eleven screens covered, against both the system back gesture and the close
  buttons. The message adapts: a single-device game is lost, a host ends it
  for everyone, a client can rejoin.
- The two screens the host also opens from the manager take a flag so that
  back there just closes them instead of offering to quit.

Translations
- 21 keys were still holding the English text in Arabic, Basque, Hindi,
  Japanese, Korean, Dutch, Polish, Russian, Turkish and both Chinese
  variants: 231 strings, covering the whole multi-device flow, permissions
  and the mode picker. Nothing failed, the app just showed English.
- The permissions dialog was hardcoded in Spanish; it is localized now.
- Portuguese used Spanish gerunds where pt-PT takes "a + infinitive".

Checked what looked untranslated but is not: Animals and Notes are correct
Catalan, Configuration and Vibration correct French, Version correct German.
Those are whitelisted in the new test rather than silently rewritten.

l10n_cobertura_test guards all of it: no missing or orphan keys, no empty
values, no English left outside the whitelist, and placeholders preserved.

lib/l10n/app_de.arb

@@ -368,5 +368,19 @@
   "reconnecting": "Verbindung wird wiederhergestellt ...",
   "reconnectingHint": "Verbindung zum Host verloren. Dein Platz im Spiel bleibt reserviert.",
   "leaveGame": "Spiel verlassen",
-  "playerRejoined": "{name} ist zurück im Spiel"
+  "playerRejoined": "{name} ist zurück im Spiel",
+  "leaveGameTitle": "Spiel wirklich verlassen?",
+  "leaveGameSingleDevice": "Das laufende Spiel geht verloren und lässt sich nicht wiederherstellen.",
+  "leaveGameHost": "Du bist der Host: Wenn du gehst, endet das Spiel für alle.",
+  "leaveGameClient": "Du verlässt das Spiel. Du kannst zurückkommen, solange der Host es offen hält.",
+  "stayInGame": "Weiterspielen",
+  "permissionsNeededTitle": "Berechtigungen erforderlich",
+  "permissionsNeededBody": "Für den Mehrspielermodus brauchen wir:",
+  "openAppSettingsQuestion": "Die App-Einstellungen öffnen?",
+  "openSettings": "Einstellungen öffnen",
+  "permissionLocation": "Standort",
+  "permissionBluetoothAdvertise": "Bluetooth (sichtbar machen)",
+  "permissionBluetoothConnect": "Bluetooth (verbinden)",
+  "permissionBluetoothScan": "Bluetooth (suchen)",
+  "permissionNearbyWifi": "Wi-Fi in der Nähe"
 }